What happened
During a manual instrument landing system (ILS) approach to runway 19L at Brisbane, the captain requested an adjustment to the flight path reference (FPR) line to match the 3.0° glideslope. To execute this, the first officer was intended to press the FPR button and adjust the selection knob. However, the first officer instead pressed the flight path angle (FPA) button.
This error caused the aircraft to disengage the ILS approach mode, switching the flight director from glideslope and localiser to flight path angle and roll modes. The unexpected change in the flight mode annunciation display startled the crew, causing them to divert their attention from monitoring the flight path to resolving the mode change. During this period of preoccupation, the aircraft descended below the glideslope and the descent rate briefly exceeded the stabilized approach limit of 1,000 feet per minute. The crew failed to recognize that the approach had become unstable and did not initiate a go-around. The descent continued until the Enhanced Ground Proximity Warning System issued a 'GLIDESLOPE' alert, yet the crew did not perform the necessary terrain avoidance maneuver and continued the approach.
The investigation
The investigation focused on the mechanics of the mode selection error and the subsequent loss of situational awareness. The ATSB identified the button press as a 'slip' error, where the pilot's intention was correct but the execution was flawed due to the close physical and functional relationship between the FPA and FPR buttons. While the aircraft manufacturer noted no significant ergonomic issues with the controls, the investigation found that the captain had not briefed the use of the FPR function during the approach, meaning the crew lacked a shared mental model for that specific task.
Findings
- The pilot monitoring mis-selected the flight path angle mode instead of the flight path reference mode.
- The crew's attention was diverted from flight path monitoring to resolving the unexpected flight mode change.
- The aircraft exceeded the glideslope deviation and descent rate limits of the stabilized approach criteria without being detected by the crew.
- The crew failed to respond to the 'GLIDESLOPE' terrain warning with the required avoidance maneuver.